Saturday, February 23, 2013

Trees on the Inside

From http://upload.wikimedia.org/wikipedia/commons/9/95/
White_lipped_tree_frog_cairns_jan_8_2006.jpg

In my herpetology class this week, we talked about the growth of herptiles (reptiles and amphibians), and we addressed one thing that I found interesting but had never fully thought about.

We all are familiar with some animals that are just really, REALLY big. For instance, there is a boa constrictor that resides in our science building on campus, and everyone can only expect him to get bigger as the years progress. I remember my childhood of watching Steve Irwin ("The Crocodile Hunter") as he would harass alligators and as Jeff Corwin caught giant toads. Oh, the wonders of Animal Planet.

The reason why herptiles continue to get bigger throughout their life is due to their growth pattern, called indeterminate growth. This means that the growth of herptiles does not end at sexual maturity, and often, the growth of these cold-blooded creatures is dependent upon food sources and the environment. Still, animals grow at the fastest rate when they are juveniles, so if you want to own a giant snake, it would be best to stuff it full of food when it is still young. Following sexual maturity, the animal needs to allocate a portion of its energy to reproductive functions. As a juvenile, the animal devotes all of its nutrients towards life and growth.

Determinate and indeterminate growth can be distinguished by bone structure. Animals with determinate growth like you and I have growth plates in our bones when we are young. As we grow, our bones lengthen, and once we have hit some point around puberty or slightly after, we are done growing up and will only grow out. If children perform strenuous activities, they may damage their growth plates and experience stunted growth, which is why some people are very cautious about children lifting weights. Animals with indeterminate growth grow more like a tree, adding a new bone layer with every growth period (which can be assumed is a year due to weather fluctuations). In other words, indeterminate growth increases the size of the entire bone, whereas determinate growth makes bones grow longer while only slightly increasing thickness in comparison.

Cross section of a wood frog toe. Notice that this one is in its
third year of growth. From http://i701.photobucket.com/
albums/ww14/mjenne1/WoodE4-6153.jpg

The tree-like growth of herptiles offers a solution to one problem of herptile enthusiasts: If an animal grows indeterminately, how can you tell how old the animal is? Size and sexual maturity really are not an accurate estimate of age. However, if you have a microscope, a knife, and a lack of regrets for amputating frog toes, you can estimate the age of a herptile by counting the rings in a cross-section of bone. It's much like a tree cookie, if you have ever had the chance to play around with those.